GEOTECHNICAL CHALLENGES OF THE LEWISTOWN NARROWS RECONSTRUCTION

The Lewistown Narrows is a heavily traveled two-lane highway conveying State Routes 0022/0322 through a narrow valley along the Juniata River in Central PA. The six-mile stretch has become a bottleneck between four-lane sections of roadway, and has been the site of numerous fatalities over the years. The Narrows has been considered for widening in the past, but the difficult terrain has prohibited a safe and cost-effective solution. In the 1990's concerns over safety and traffic congestion renewed the urgency for widening the roadway, and a design team was assembled to come up with a solution.

The geologic structure of the area plays a key role in the difficulties inherent in the project. The most prominent geologic feature borders the existing roadway to the north, and consists of a marginally stable talus slope comprised of sandstone boulders weathered from the Tuscarora Formation. To the south, the project is constrained by the Juniata River and the historic PA Canal. Design concerns include the fact that traffic must be maintained on the roadway during construction, the limited accessibility for construction equipment, and the potential impact of construction vibrations on the talus.

The design of the roadway widening included several unique features to safely accommodate site constraints. Features include the extensive bifurcation of the roadway, in some cases elevating the proposed roadway over the talus to avoid cutting into the slope. “Pin piles” were used to provide resistance to slope failure. The design incorporates reinforced soil slopes, mechanically stabilized earth walls, steel H-piles, concrete cantilever walls, vibration monitoring, and temporary and permanent rockfall protection ditches/fences. The design utilizes rock anchors to retain Tuscarora sandstone having beds dipping toward proposed roadway cuts and with clay “slip” layers and numerous springs.

The roadway construction commenced in 2004, and is scheduled for completion in 2008. The construction contract was the second largest awarded in Pennsylvania Department of Transportation (PADOT) history. Some PADOT officials are calling the Narrows the most complex highway segment ever constructed in Pennsylvania.